An electric current measuring apparatus is known in which the Faraday effect of an optical fiber is used and which has various advantages, such as small size, flexibility, electromagnetic noise resistance, long-distance signal transmission, and withstand voltage, and the like. As an example of the electric current measuring apparatus, Patent document 1, WO 2006/022178 discloses a reflective electric current measuring apparatus which uses the Faraday effect, that is, uses rotation of a polarization plane of a light by action of a magnetic field.
This electric current measuring apparatus is a reflective electric current measuring apparatus which uses a lead-glass fiber as a optical fiber for a sensor, and in which a mirror is disposed at the other end of the optical fiber for a sensor. The optical fiber for a sensor is provided around the periphery of an electric conductor, through which measured electric current flows, to detect the measured electric current. Basically, while a linearly polarized light incident from one end of the optical fiber for a sensor travels to the mirror and returns, a Faraday rotation angle of the linearly polarized light rotated by a magnetic field of the measured electric current is detected. The electric current measuring apparatus also includes a Faraday rotator made of a ferromagnetic crystal such as YIG.
However, the Faraday rotation angle of the Faraday rotator used in the electric current measuring apparatus has a characteristic (temperature characteristic) depending on ambient temperature. The optical fiber for a sensor also has a temperature characteristic of a ratio error resulting from the temperature dependency of a Verdet constant and the Faraday rotation angle. Thus, it is necessary to perform not only compensation in the Faraday rotator but also compensation for (reduction in) the temperature characteristic of the optical fiber for a sensor. As an electric current measuring apparatus which fully compensates for the temperature characteristics of both the Faraday rotator and the optical fiber for a sensor, the present inventors invented an electric current measuring apparatus which set a fluctuation range of a ratio error within ±0.5% or ±0.2% (refer to Patent Literature 2).
The electric current measuring apparatus described in Patent document 2, JP 2010-271292A is configured to include at least a optical fiber for a sensor, a polarization splitter, a Faraday rotator, a light source, and a signal processing circuit including a photoelectric conversion element. The optical fiber for a sensor is provided around the periphery of an electric conductor through which measured electric current flows. Also, a Faraday rotation angle of the Faraday rotator at the time of magnetic saturation is set to 22.5°+α° at a temperature of 23° C. to change the Faraday rotation angle as much as α° from 22.5°, such that a fluctuation range of a ratio error in a measurement value of the measured electric current output from the signal processing circuit is set within a range of ±0.5% or ±0.2% in a temperature range between −20° C. or higher and 80° C. or lower.
By reducing the fluctuation range of the ratio error of the Faraday rotator, the fluctuation range of the ratio error in the measurement value of the measured electric current output from the signal processing circuit is suppressed to be within ±0.5% or ±0.2%. Therefore, the compensation for the temperature characteristic of the ratio error in the measurement value can be performed in the Faraday rotator, thereby improving reliability of the electric current measuring apparatus. Also, by suppressing the fluctuation range of the ratio error to be within ±0.5% or ±0.2%, it is possible to achieve the electric current measuring apparatus which can be applied to a protection relay.